The fabrication of luminescent glasses doped with metal ions and, in particular, copper ions has been of significant interest for many years. In the early 1980's luminescent copper-doped glasses were investigated for their potential utility as laser gain media. In addition to high luminescent quantum yield, the glasses studied required macroscopic dimensions (mm-cm size range) and excellent optical quality (minimal scattering and/or absorption losses).
Copper can exist in glass in at least three oxidation states: Cu0, Cu+1 and Cu+2, but only Cu+1-doped glass was considered for laser development because Cu+1 is the only ion that does not introduce unwanted absorption bands at the visible wavelengths of interest. Copper doping of glasses other than silica glass, including for example, silicate, borate, and phosphate glasses, has typically been accomplished by mixing the component raw materials in a crucible, heating the components until they melt, and then cooling the liquid mixture to form a glass [Zhang, 1990; Boutinaud, 1991; Tanaka, 1994; Sharaf, 1994]. The final oxidation state of the copper dopants in the glass was found to be critically dependent on the composition of the mixture and the melting parameters, such as the temperature and the environment. Mixtures melted in air (an oxidizing environment) favored the formation of Cu+2 ions. However, the Cu+2 ions were characterized by broad absorption bands in the visible that caused the glass to appear colored, and the Cu+2 absorption represented a significant optical loss that could prevent laser operation. Cu+1 doping is preferred because it provides an optically clear glass since the absorption band of Cu+1 is in the ultraviolet. However, the fabrication of Cu+1-doped glass is more difficult than Cu+2-doped glass [Boutinaud, 1991]. Reducing conditions must be used during the glass melting step in order to favor the formation of Cu+1. If the reducing conditions are not strong enough, then even small concentrations of residual Cu+2 ions can interfere with laser operation because the Cu+2 ions have an ultraviolet absorption band (in addition to the broad visible absorption band).
The ultraviolet absorption of the Cu+2 ions overlap with the ultraviolet absorption band of the Cu+1 ions, and thus competes with the optical excitation of the Cu+1 ions [Tanaka, 1994]. On the other hand, if the reducing conditions are too strong, then colloidal crystals of Cu0 can form. These colloidal crystals can cause the glass to have a ruby red color with low concentrations [Sharaf, 1994]. Higher concentrations of the colloidal crystals can cause the color of the glass to become black [Tanaka, 1994]. Even if the reducing conditions are carefully controlled, i.e., by the addition of NH4H2PO4 in order to provide a reducing environment, it was found that these chemicals decomposed and interfered with the Cu+1 emission of the glass [Tanaka, 1994].
The current state of the art for the fabrication of optically transparent, luminescent copper-doped silicate-based glasses that contain only monovalent copper is still problematic. The technical problems associated with obtaining only Cu+1 doping, as outlined very briefly above, have not been solved.
Other researchers have attempted to fabricate luminescent glasses containing only Cu+1 ions by doping copper into silica glass, but have had to address similar technical difficulties [Debnath 1989; Chaudhuri, 1994; Fujimoto, 1997; Garcia, 2001; Brownlow, 1981]. Debnath and Das [Debnath, 1989] prepared luminescent, Cu+1-doped silica glass by soaking pieces of porous silica glass in solutions (presumably aqueous, but not specified) of a copper complex (again, the starting chemical was not specified). The porous host glass is not pure silica, but is typically ˜96% silica (e.g. Corning Vycor® glass). The impregnated glass was allowed to dry and then was sintered at 1000 C-1200 C under an inert gas environment. The copper-doped glass prepared by this method was shown to be radiation sensitive, but the sensitivity was quite low [Debnath, 1995], indicating that the glass had a low concentration of trap and/or luminescence centers.
Fujimoto and Nakatasuka [Fujimoto, 1997] fabricated luminescent, copper-doped silica glass using a sol-gel technique. It is generally known [Fujimoto, 1997] that dopants are not readily dispersed homogeneously in silica glass. For example, the homogeneous dispersion of Nd ions in silica is quite difficult, and the Nd ions will agglomerate in silica glass at low concentration. Dispersion of the Nd ions is aided by the addition of a co-dopant ion, such as aluminum. Fujimoto mixed tetraethyl-orthosilicate hydrolyzed with hydrochloric acid, with colloidal silica. He then added an ethanol solution of copper chloride and uniformly mixed it. In this approach, the copper ions are uniformly dispersed before the glassy network is formed. The gel was dried and then sintered. The resulting luminescent, copper-doped silica glass contained both Cu+1 and Cu+2 ions. Garcia also used a sol-gel fabrication method to prepare luminescent, thin films of Cu+1-doped silica [Garcia, 2001]. Brownlow and Chang reported [Brownlow, 1981] the fabrication of luminescent Cu+1-doped silica glasses using a method whereby silicic acid was mixed with copper and aluminum nitrates in water and then reacted at 1200 C in wet nitrogen gas. The authors could not explain why aluminum, gallium or hydroxyl co-dopants were required to activate the Cu+1 luminescence in the silica glass host.
Photoluminescent silica glasses were prepared by doping silica glass with copper ions via ion implantation [Fukumi, 1998]. Copper ions were implanted to a depth of less than a micrometer from the surface using an accelerator operating at 2 MeV. X-ray absorption fine structure spectroscopy was performed in order to study the structure and bonding of the copper-doped silica. Fukumi found that the oxidation state of the copper was Cu+1 and that each copper ion was coordinated by two oxygen atoms. Fukumi reported that co-implantation of oxygen ions stabilized the Cu+1 ions in the silica glass and increased the photoluminescence intensity.
Copper-doped silica glasses have also been fabricated and used for applications besides the development of laser gain media. For example, [Huston, 1998] describes the development of copper-doped glasses that were used for the detection of ionizing radiation using optically stimulated luminescence (OSL) and thermoluminescence (TL). In the description, above, of the development of copper-doped glasses for use as potential laser gain media, the required properties of the glasses included high Cu+1 photoluminescence yield, low background absorption due to Cu+2 ions or other additives, and the ability to make bulk-size samples with homogeneously dispersed dopants. Copper-doped glasses for the detection of ionizing radiation by OSL or TL have similar requirements, except that maximizing the photoluminescence yield is not critical. A material with sensitive OSL and/or TL properties must possess a high concentration of trapping centers that are capable of storing trapped charges for long periods of time, in addition to luminescence centers that exhibit photoluminescence. The copper-doped silica glasses described by Huston [Huston, 1998] were extremely well-suited for detection of radiation using optically stimulated luminescence (OSL) and thermoluminescence (TL) methods and therefore possessed significant populations of trapping centers. In one embodiment, fused silica glasses were doped with Cu+1 using a novel thermal diffusion method that involved coating the silica glass with a thin layer of copper sulfide-doped sol-gel glass and then heating the coated glass at temperatures high enough to cause diffusion of copper atoms into the silica glass. Another embodiment of the thermal diffusion method [Huston, 2000] required first doping porous Vycor glass powder with metal ion dopants and then using that doped powder as the source of copper in the thermal diffusion of dopants in fused silica glass. In this procedure, the porous Vycor glass powder was immersed in a solution of CuSO4, dried, and then exposed to H2S to create CuS in the pores of the glass powder. The H2S was provided directly or by the decomposition of thioacetamide. The doped Vycor powder was packed around a fused silica glass sample and heated to a temperature of 1100 C. The Cu+1 ions dope the fused silica glass by thermal diffusion.
Both of these thermal diffusion methods yielded extremely low concentrations of Cu+1 ions in the glass (Huston referred to the methods as “seasoning” the glass). However, it was found that very small amounts of Cu+1 ions in the glass were sufficient “to cause significant and useful OSL activity.” It was apparent that, even at extremely low dopant concentration, the doped glasses were capable of storing trapped charges and the trapped charges could be released in response to an optical or thermal stimulation, resulting in OSL or TL. Although these copper-doped glasses were well-suited for OSL and TL methods of radiation detection, their utility for scintillator applications was limited to very specific applications because their radioluminescence yields were less than those of most other inorganic and organic scintillators. As an example, an application that required the detection of radioluminescence from the copper-doped glass was the gated detection of radiation produced by a medical linear accelerator [Justus, 2004]. An optical fiber dosimeter was developed that used a copper-doped glass fiber sensor. The gated detection technique succeeded, despite the extremely low radioluminescence signals, because highly sensitive photon counting modules were used to detect the weak signal. In addition, the thermal diffusion methods described above yielded doped glasses with significant spatial concentration gradients and/or inhomogeneities. The concentration gradients were presumably caused by depletion of the copper atoms in the sol gel film and/or the Vycor powder.